Originally, keywords were the beginning and end of SEO; picking the right keywords was absolutely necessary, and you could deliberately increase your rankings for those specific keywords, monitoring your progress with great precision.

These days, the keyword game has been complicated by a handful of different factors:

Google’s Hummingbird update has radically changed the way Google views keywords (not to mention content). Now, the search engine focuses on the semantic context of a user query more than the specific words and phrases it calls upon, making it hard to achieve one-to-one momentum by optimizing for specific keywords.

The lack of precise ranking information makes it difficult to exactly determine your rankings. Google no longer provides this information, and even though some third-party sites offer it on their own, it’s still tough to compensate for all the personalization that happens behind the scenes for individual users (such as user preferences, search history, device used to perform the search, geographic location, and whether or not they’re logged into their Google account).

The volatility of rankings meddles with the potential consistency of a keyword strategy. Google changes things constantly, and your competitors will be optimizing alongside you in real time.

Still, keywords are an important tool for SEO and content marketing campaigns. Keyword research gives you excellent insights into user search patterns (not to mention knowledge about your target audience), helps guide the topics and focus of your content marketing campaign, and of course, gives you a solid metric with which you can measure your performance.

In my keyword research guide, I outline a step-by-step process for exactly how to conduct keyword research for SEO or content marketing campaigns. Here, I’ll outline six of my favorite tools to use for keyword research.

1. Google's Keyword Planner.

No keyword tool list would be complete without Google’s Keyword Planner, which is a must-have tool for three reasons: it’s straight from the source (Google), it’s free, and it’s specifically designed to give you accurate keyword information. The only catch is that it’s designed for paid advertising campaigns (as opposed to SEO or content marketing campaigns)—but you can easily work around that. Enter in a handful of phrases relevant to your brand, and you’ll be able to determine not only the search volume for those keywords, but also the relative amount of competition that’s currently fighting over them.

2. Moz's Keyword Explorer.

Moz has always offered exceptional tools for SEO (and free ones at that), and one of their latest, the Keyword Explorer, is no exception. Though a few of the metrics you’ll pull with this tool are similar to those you’ll find in Google’s Keyword Planner, these are purported to be more accurate, as Google rounds its data and isn’t as thorough when it comes to rating the competition. Also, Moz’s tool will give you more detailed keyword recommendations and will qualitatively rate the cumulative value of each keyword you enter.

3. Ubersuggest.

Ubersuggest isn’t as robust as Google’s or Moz’s tools, but it doesn’t need to be. Google and Moz excel at giving you information for keywords and phrases you enter, but Ubersuggest’s strength lies in helping you come up with more keyword ideas. Relying on the same system that powers Google’s own keyword suggestions, simply enter any query into Ubersuggest’s engine and you’ll be met with a list of dozens of keyword suggestions. These won’t be recommended based on search volume, competition, or potential value—but they will help you come up with more specific, relevant, diversified ideas for your campaign. It’s a must-have tool in the beginning of your campaign, especially if you’re inexperienced with keyword research to begin with.

4. Soovle.

Soovle takes a similar approach as Ubersuggest, but with one critical difference; it relies on the auto-suggest features of several different engines at once, including Google, Wikipedia, YouTube, Amazon, Bing, Yahoo, and Answers. You won’t find the sheer quantity of recommendations here that Ubersuggest will offer, but there’s no beating its diversity. This is especially useful if you plan on marketing your content or brand on different platforms, such as listing your products on Amazon or developing your own YouTube channel.